As the trend of portable electronics toward multifunctionality has been outpacing the trend toward reduction in power requirements for electronic components, power consumption of portable electronics has been on the rise. Because of this, lithium-ion batteries, which are the primary power sources in portable electronics, are in demand more strongly than ever to have larger capacities and be smaller in size. In addition, with the growing demand for electric vehicles, lithium-ion batteries for use in such vehicles are strongly demanded to have larger capacities.
Graphite is mainly used as a negative electrode material for the conventional lithium ion batteries. Graphite can occlude Li only up to the stoichiometric ratio of LiC6. Therefore, the theoretical capacity of a lithium ion battery in which graphite is used as a negative electrode is 372 mAh/g.
In order to achieve a high capacity of a lithium ion battery, particles comprising a metal element, such as Si, Sn or the like, having a large theoretical capacity are considered to use for a negative electrode material. For example, the theoretical capacity of a lithium ion battery in which Si-containing particles are used for a negative electrode material is 4200 mAh/g. In contrast, the theoretical capacity of a lithium battery in which metal lithium is used is 3900 mAh/g. Therefore, if Si and the like can be used for a negative electrode material, a lithium ion battery which is smaller and of a higher capacity than a lithium battery is expected to be obtained. However, a negative electrode material such as Si or the like has a large expansion and contraction associated with intercalation and deintercalation (occlusion and release) of lithium ions. This may result in gaps between particles, and a capacity as good as expected can not be obtained. Further, since particles crack into fine powders due to repeated large expansion and contraction, electric contacts may break to increase internal resistance. Therefore, the resulting lithium ion battery may have a short charge and discharge cycle life.
Accordingly, the followings have been proposed such as a negative electrode material comprising Si and/or Sn-containing particles and fibrous carbon (Patent Document 1); a negative electrode material comprising a graphite particle and a carbonaceous material attached to the surface of the graphite particle, the carbonaceous material containing a Si-containing particle and fibrous carbon (Patent Document 2); a negative electrode material comprising a mixture of metal-based particles such as Si, Sn, Ge or the like and graphite particles in which d002 is not less than 0.3354 nm and not more than 0.338 nm and the area ratio of the G peak and the D peak by the Raman spectroscopy is G/D≧9 (Patent Document 3); a negative electrode material comprising a solid solution comprising an element such as Si, Ge or the like capable of occluding and releasing lithium ions and an element such as Cu or the like incapable of occluding and releasing lithium ions (Patent Document 4); a negative electrode material in which Si particles are attached to the surface of a graphite particle, and a carbon coating is covered on at least a portion of the surface of the graphite particle (Patent Document 5); an electrode structure in which a metal powder and a supporting powder are compounded with a binding material serving to form chemical bonds (Patent Document 6).